Electrical power cables are ubiquitous and used for distributing power across vast power grids or networks, moving electricity from power generation plants to the consumers of electric power. Power cables characteristically consist of a conductor (typically copper or aluminum and typically multi-stranded tube) and may be surrounded by a semiconductor and one or more layers of insulating material. Metal wires may be wound helically around the semiconductor to serve as ground wires and a cable jacket surrounds the entire construction to protect the electrical cable. Power cables may be constructed to carry high voltages (greater than about 50,000 Volts), medium voltages (between about 1,000 Volts and about 50,000 Volts), or low voltages (less than about 1,000 Volts).
As power cables are routed across the power grids to the consumers of electric power, it is often necessary or desirable to periodically terminate the electrical cable for making a connection to electrical equipment. Typically, a termination is used to make electrical connection between the insulated electrical cable and an unshielded, un-insulated conductor. The terminator fits over an end of the insulated cable.
When a power cable is terminated, the conductor is exposed by removing some predetermined length of the cable jacket and some predetermined length of the cable semiconductor. Typically the ground wires are collected and gathered around the cable jacket to ground the semiconductor. The separation distance between the insulator and the semiconductor provides creepage distance from the live conductor, which is at 100% potential, and the grounded semiconductor, which is at 0% potential.
A termination of the electrical cable creates an abrupt discontinuity in the electrical characteristics of the cable. The termination also exposes the cable insulation to ambient conditions that most likely contains gases, moisture, and particles. The exposed conductor is also susceptible to corrosion. The discontinuity of the cable's semiconductor layer increases the maximum voltage gradient (in volts per distance, such as volts per inch) of the insulation at the semiconductor end. The discontinuity also changes the shape of the resulting electrical field and electrical stress so as to increase the risk of insulation breaking down. Thus, one function of a terminator, among others, is to compensate for the change in electrical field and electrical stress generated when there is a discontinuity in the electrical cable. The terminator also functions to protect the terminated end portion from the ambient conditions.
There are two general classes of terminators, the “wet” type and the “dry” type. In the wet type terminator, the insulating body typically contains a stress relief element, applied at the terminated end of the semiconductor layer. A suitable dielectric material, such as oil, typically fills the cavity between the cable and the inside wall of the insulating body. In the dry type terminator, the insulation body typically contains a stress relief element having an inside diameter that provides an interference fit over the cable insulation and typically over the cable semiconductor. There are two general classes of stress relief elements for use with wet or dry type terminators: (1) capacitive type stress relief element and (2) geometric type stress relief element.
A capacitive type stress relief element can be constructed from a non-compressible elastomer and is generally cylindrical tube in design. The capacitive type stress relief element relies primarily on the material selection to manage the electrical field and the electrical stresses resulting from a terminated electrical cable. A useful material should be a good insulator and have a large dielectric constant. For example, for a medium voltage (e.g., 15 kV) electrical termination, the dielectric constant for the capacitive stress relief element should be greater than about 12. For a high voltage (e.g., 69 kV) electrical termination, the dielectric constant should be greater than about 20. While the capacitive type stress relief element (often referred to colloquially as “high K tube”) are useful in the low voltage and medium voltage application, they are less effective in high voltage applications. Although high K tubes are commercially available for 69 kV termination system, it is commonly understood by those skilled in the art that at the high voltages, the high K tube wall tends to rupture due to the electrical stresses.
A geometric type stress relief element relies on its geometric design as well as the material type to manage the electrical field and electrical stresses resulting from a terminated cable. In one design, the geometric stress relief element is conical in shape and contains a semiconductor electrode embedded in an insulator.
Cold-shrink technology has been used to deliver capacitive stress relief elements. For example, high K tubes have been preloaded to cold shrink tubes for 15 kV, 39 kV, and 69 kV termination systems. For example, for a 69 kV system, the capacitive type stress relief element, such as a high K tube made of EPDM having a dielectric constant of about 11 to 25, can be about 0.200 inch (5 mm) thick. The length of the high K tube is typically determined by the dielectric constant of the tube. As one skilled in the art will recognize, there are commercially available cold shrink tubes can support a 0.200 inch thick high K tube.
In high voltage electrical cables, the size of the various parts used to terminate the cable can increase considerably compared to that of the medium or low voltage terminators. This increase in size is particularly true for a geometric type stress relief element. With larger stress relief elements, it becomes more difficult to use a cold shrink tube to deliver them to the termination because of the increased compressive stress that is imposed on the tube.
The current field installation method for a geometric type stress relief element to a terminated electrical cable requires the efforts of several people and requires the use of a specialized equipment, such as a come-a-long. In a typical process, the terminated cable is lubricated and the geometric type stress relief element is pushed on to the lubricated, terminated cable, with the use of the come-a-long. This installation method is tends to be labor intensive and can be prone to installation error.
The termination also will typically contain a plurality of skirts. Traditionally, the skirts are premolded with an insulator and the combination is installed on a termination. For example, a termination that uses a porcelain housing will typically contain a predetermined number of premolded porcelain skirts to increase the distance from the top to the bottom of the termination.
Thus, there is a need to advance the installation process of geometric type stress elements to terminated high voltage electrical cables. And, there is also a need to move away from a predetermined number of skirts to give the user flexibility in installing the desired number of skirts needed to achieve a desired impulse performance for the specified voltage class.